Secondary batteries that are repeatedly chargeable and dischargeable are used in various electronic devices. As types and shapes of electronic devices have become various, secondary batteries mounted on electronic devices have also been diversified in shape. Recently, lithium ion secondary batteries using lithium as secondary batteries have been widely used.
To manufacture such a secondary battery, a negative electrode coated with a negative electrode active material, a positive electrode coated with a positive electrode active material, and a separator disposed between the negative electrode and the positive electrode are required. Particularly, to prepare the negative electrode active material and the positive electrode active material, which constitute the secondary battery, a mixture of an electrode active material, a conductive agent, and a binder, which is called slurry, is often required.
To prepare such slurry, a process of mixing powder and a solvent is required, and the mixing occurs in a mixer. Particularly, to prepare the slurry, a mixing ratio of the powder and the solvent is important. Thus, the mixing ratio of the powder and the solvent, which is performed for preparing the slurry, is called a kneading point. In the slurry preparation step, the powder and the solvent are mixed in the mixer for a predetermined time at the kneading point and then used as a material for the secondary battery.
However, the kneading point may vary according to materials and contents of the powder and the solvent, which are necessary for preparing the slurry. Also, the kneading point may vary according to a size and structure of the mixer that is used for preparing the slurry.
When the slurry is manufactured in the mixer, a worker often checks the inside of the mixer so as to determine the kneading point according to the related art. In this process, external impurities may be frequently introduced into the mixer, and thus, the slurry be deteriorated in quality. In addition, it is difficult to determine an optimum kneading point because an error surely occurs due to the direct check of the inside of the mixer by the human worker.